TL;DR: Red welts on dogs are typically hives caused by an allergic reaction; while often mild, you must seek emergency care if they are accompanied by facial swelling or difficulty breathing.
What causes sudden red welts or hives on my dog's skin?
- Red, raised welts, known as hives or urticaria, are localized areas of swelling that occur when the immune system overreacts to an allergen.
- Common triggers include insect stings, vaccinations, certain plants, or new foods.
- The welts can vary in size and often appear suddenly across the torso, face, or legs, sometimes causing the hair to stand up in patches.
When are red welts on my dog considered a veterinary emergency?
- The urgency level for sudden welts is Medium; while often mild, they can progress to life-threatening anaphylaxis.
- Seek emergency care immediately if welts are accompanied by muzzle or eye swelling, labored breathing, persistent vomiting, or sudden weakness.
- If your dog is acting normally, contact your veterinarian to discuss the appropriate use of antihistamines.
Why should I take a photo of my dog's hives before heading to the clinic?
- Hives can change shape, migrate, or disappear entirely by the time you reach a veterinarian.
- A clear photo or video provides a visual record of the reaction at its peak severity.
- Visual evidence helps the medical team differentiate hives from other conditions like bacterial infections or heat rash.
Clinical Context (Merck Veterinary Manual)
Urticaria, characterized by raised, round, flat-topped wheals or plaques ranging from 0.5 to 20 cm in diameter, can appear rapidly on any part of the body, commonly the back, flanks, neck, eyelids, and legs, and in advanced cases, mucous membranes. Severe cases may be preceded by fever, anorexia, or dullness, with excitement and restlessness. Angioedema, a life-threatening variant, involves diffuse subcutaneous edema, often localized to the head, limbs, or perineum. In severe cases of urticaria due to insect stings, epinephrine should be administered intramuscularly; anaphylaxis requires immediate subcutaneous administration of epinephrine (1:1,000, 0.1-0.5 mL), repeatable every 10-20 minutes, or intravenous administration (1:10,000, 0.5-1 mL) with vigilant monitoring of heart rate, rhythm, and blood pressure. Intravenous fluids are indicated to prevent vascular collapse.
Chapter: Toxicology, Emergency
Source: The Merck Veterinary Manual, 11th Edition (Page 3164)